Field of the Disclosure
The present application relates to a solid-state laser that generates light near 193 nm and is suitable for use in photomask, reticle, or wafer inspection.
Related Art
The integrated circuit industry requires inspection tools with increasingly higher resolution to resolve ever smaller features of integrated circuits, photomasks, solar cells, charge coupled devices etc., as well as detect defects whose sizes are of the order of, or smaller than, feature sizes. Short wavelength light sources, e.g. sources generating light under 200 nm, can provide such resolution. Specifically for photomask or reticle inspection, it is desirable to inspect using a wavelength identical, or close, to the wavelength that will be used for lithography, e.g. substantially 193.368 nm, as the phase-shifts of the inspection light caused by the patterns will be identical or very similar to those caused by the same patterns during lithography. However, the light sources capable of providing such short wavelength light are practically limited to excimer lasers and a small number of solid-state and fiber lasers. Unfortunately, each of these lasers has significant disadvantages.
An excimer laser generates an ultraviolet light, which is commonly used in the production of integrated circuits. An excimer laser typically uses a combination of a noble gas and a reactive gas under high pressure conditions to generate the ultraviolet light. A conventional excimer laser generating 193 nm wavelength light, which is increasingly a highly desirable wavelength in the integrated circuit industry, uses argon (as the noble gas) and fluorine (as the reactive gas). Unfortunately, fluorine is toxic and corrosive, thereby resulting in high cost of ownership. Moreover, such lasers are not well suited to inspection applications because of their low repetition rate (typically from about 100 Hz to several kHz) and very high peak power that could result in damage of samples during inspection. Furthermore, high-speed inspection typically requires minimum laser pulse repetition rates of multiple MHz (e.g. greater than 50 MHz in some cases) in order to allow high-speed image or data acquisition with low noise.
A small number of solid state and fiber based lasers producing sub-200 nm output are known in the art. Unfortunately, most of these lasers have very low power output (e.g. under 60 mW), or very complex design, such as two different fundamental sources or eighth harmonic generation, both of which are complex, unstable, expensive and/or commercially unattractive.
Therefore, a need arises for a laser capable of generating 193 nm light yet overcoming the above disadvantages.